R4/P5 hard to find the original parts as these are from the NHRA Pro Stock Truck days.

This engine could be run in NHRA Comp Eliminator or NHRA Super Stock in Modified, or with a single 850 or 750 carb Modified Stock.

Most of them seem to be 358 CI.

You can build one new with a Ritter block and Slawco heads.

Patterson-Elite maintains them for a Comp racer and an Super Stock racer as I have seen the engines (in crates) there.

Edit, just realized this is two engines, one with the drag race (Hemi heads) and one with what I call the NASCAR heads. Not sure if those were drag raced on the trucks or not. Maybe someone else knows the history better.